About a week ago I was working on a paper about, among other things, Flannery O'Connor's "A Good Man Is Hard To Find." I found myself using the possessive form of "Jesus" in this paper and MS Word took issue with my decision to type "Jesus's." This spelling is correct according to every grammar lesson I've ever had on the topic of possesives. I did some looking around and found out that I was in fact wrong. The generally accepted form is Jesus' which Wikipedia tells me is considered “an accepted liturgical archaism” by Hart's Rules. I love the little idiosyncrasies found in the English language.
Classical, biblical, and similar names ending in a sibilant, especially if they are polysyllabic, do not take an added s in the possessive; among sources gtiving exceptions of this kind are The Times and The Elements of Style, which make general stipulations, and Vanderbilt University, which mentions only Moses and Jesus. As a particular case, Jesus' is very commonly written instead of Jesus's – even by people who would otherwise add 's in, for example, James's or Chris's. Jesus' is referred to as “an accepted liturgical archaism” in Hart's Rules.